1. Field of the Invention
The present invention relates to a molten-salt type catalyst which can efficiently purify particulate materials (hereinafter abbreviated to as xe2x80x9cPMxe2x80x9d), contained in an exhaust gas emitted from an internal combustion engine, such as a diesel engine, etc., in a range of exhaust gas temperature.
2. Description of the Related Art
xe2x80x9cPMxe2x80x9d are contained in an exhaust gas which is emitted from an internal combustion engine. The xe2x80x9cPMxe2x80x9d are composed of carbon, xe2x80x9cSOFxe2x80x9d (i.e., soluble organic fractions), polymerized organic compounds, sulfuric acid mists, and so on. In view of air pollution and adverse effects to human bodies, there arises a growing tendency toward the suppression of xe2x80x9cPMxe2x80x9d emission. In order to control the xe2x80x9cPMxe2x80x9d emission, there are two ways. One of them is to catch xe2x80x9cPMxe2x80x9d with a filter. The other one of them is to burn and remove xe2x80x9cPM.xe2x80x9d The engineering developments have been carried out in which the two ways are applied independently or in which the both of them are combined.
In order to burn xe2x80x9cPM,xe2x80x9d it is possible to think of using an oxidizing catalyst on which a noble metal, for example, Pt, and the like, is loaded. In this case, however, since the solid phase of xe2x80x9cPMxe2x80x9d and the solid phase of the oxidizing catalyst contact with each other, xe2x80x9cPMxe2x80x9d and the oxidizing catalyst contact with each other with a low probability. Accordingly, it is difficult to efficiently burn and purify xe2x80x9cPM.xe2x80x9d In particular, the noble metal, such as Pt, hardly effects to appreciably burn and purify carbonaceous components, which are contained in xe2x80x9cPM.xe2x80x9d
Hence, it was thought of turning a catalyst into a liquid phase and then contacting it with xe2x80x9cPM.xe2x80x9d For instance, in Appl. Cat. B21 (1999) pp. 35-49, there is a report on molten-salt type catalysts, such as Cs2MoO4xe2x80x94V2O5, CsVO3xe2x80x94MoO3 and CS2SO4xe2x80x94V2O5. Such molten-salt type catalysts are used under the favorable conditions that they exhibit a high oxidizing capability, they evaporate less and the combustion temperature is low. In this literature, there is a description that Cs2MoO4xe2x80x94V2O5 and CsVO3xe2x80x94MoO3 were preferable catalysts because they exhibited high activities at 620 K (i.e., 347xc2x0 C.) or more and were stable up to 1,025 K (i.e., 752xc2x0 C.) in air.
Moreover, in Japanese Unexamined Patent Publication (KOKAI) No. 9-144,528, there is set forth a catalytic apparatus. In the catalytic apparatus, a eutectic composition, such as Cs2Oxc2x7V2O5 and K2O5xc2x7V2O5, is loaded on a carrier, such as a monolithic substance. The eutectic composition has a melting point of from 300 to 500xc2x0 C., and exhibits a catalytic activity in the combustion of soot.
In addition, in a Patent Cooperation Treaty International Publication No. W000/43,109 (PCT/JP00/00194), there is set forth that xe2x80x9cPMxe2x80x9d can be burned and purified by putting magnesium nitrate and magnesium carbonate in a liquid reservoir and contacting them with an exhaust gas at a temperature of from about 185 to 270xc2x0 C.
A temperature of an exhaust gas, which is emitted from a diesel engine, is usually less than 300xc2x0 C. Moreover, when a diesel engine is started, the temperature is much lower. Accordingly, when a molten-salt type catalyst has a melting point of 300xc2x0 C. or more, it is not turned into a liquid phase in an exhaust gas which is emitted from a diesel engine. Consequently, it is difficult for such a molten-salt type catalyst to efficiently burn and remove xe2x80x9cPM.xe2x80x9d
It is required that a molten-salt type catalyst be turned into a liquid phase. However, since it is turned into a liquid phase, there is a fear in that it is flowed to and is concentrated on a downstream side when it contacts with an exhaust gas flow. Hence, in the technology disclosed in Patent Cooperation Treaty International Publication No. W000/43,109, such a primitive technique as the liquid reservoir is employed. However, from the viewpoint of boarding the liquid reservoir onto an automobile, it is not practical because the liquid level is made unstable by increments of exhaust gas pressure, vibrations during driving, and so on.
The present invention has been developed in view of the aforementioned circumstances. It is therefore an object of the present invention to make a practical molten-salt type catalyst which can burn and remove xe2x80x9cPMxe2x80x9d even in a low temperature range, and which can be stably disposed in an exhaust system of an automobile.
A molten-salt type catalyst according to the present invention can achieve the aforementioned object. The present molten-salt type catalyst is adapted for purifying particulate materials, which are contained in an exhaust gas emitted from an internal combustion engine and containing carbon, and comprises: a solid support; and a catalytic ingredient loaded on the solid support including at least one member selected from the group consisting of silver nitrate, alkali metal nitrate, alkaline-earth metal nitrate and rare-earth nitrate.
The solid support can preferably be a basic support. Moreover, the catalytic ingredient can preferably include alkali metal nitrate. In addition, the catalytic ingredient can more preferably further include an oxidation facilitating ingredient.
Thus, in accordance with the present invention, not only it is possible to burn and remove xe2x80x9cPMxe2x80x9d even in a low temperature range, but also it is possible to stably dispose the present molten-salt type catalyst in an exhaust system of an automobile. Hence, the present molten-salt type catalyst is an extremely practical one.